Conformational flexibility and kinetic complexity in antibody-antigen interactions

The kinetics of dissociation of three structurally characterized anti-hen egg white lysozyme antibodies (H8, H10, and H26), with hen egg white lysozyme (HEL) and the avian variant Japanese quail lysozyme (JQL) were examined. These antibodies share over 90% sequence identity and recognize the same epitope, but differ in their degree of cross-reactivity and predicted combining site rigidity. Competitive dissociation induced by the addition of excess unlabeled HEL after varied periods of antibody-antigen association was followed in real time using fluorescence anisotropy. Dissociation was in many cases non-single-exponential, and the observed off-rates became slower as the complex age increased, suggesting multi-step association kinetics consistent with an encounter-docking view of protein-protein interactions. The fully docked fraction of the complexes just prior to inducing dissociation was high for the HEL complexes but was dramatically reduced for JQL complexes, that is final docking was antigen-sensitive. Variations among the systems can be understood in terms of the complexes' differing conformational flexibilities, based on the encounter-docking model of protein-protein associations.